In a system of electronic devices such as smartphones, a processor and various devices, e.g., a display device, a touch sensor device, a communication device, and a memory device, are connected via interfaces that correspond to each device.
Such a system stores setting information in a setup register in an interface and then operates (see Patent Document 1, for example). This operation is referred to as initialization. This initialization is required for interfaces each corresponding to a display device, a touch sensor device, a communication device, a memory device, or the like. Note that setting information is information on settings, such as the presence or absence of a target device, a type of the device, specifications of the device, and a driving method of the device. Such a configuration can increase the extensibility and versatility of the system. After the initialization, the system can operate normally.
For display devices, a structure using a transistor including an oxide semiconductor in a channel formation region (OS transistor) in a pixel is proposed (see Patent Document 2, for example). An OS transistor have an extremely low leakage current in an off state (off-state current), and thus can reduce the rewriting frequency (refresh operation) of video signals on the basis of image data in displaying a still image in order to reduce power consumption.